1. Field of the Invention
The subject invention relates to fuel pumps for gas turbine engines, and more particularly, to vane pumps which are used in applications that require high operational reliability and a predicted failure mode.
2. Background of the Related Art
Vane pumps are being developed within the aerospace industry as an alternative to traditional gear pumps. An example of a variable displacement vane pump is disclosed in U.S. Pat. No. 5,545,014 to Sundberg et al., the disclosure of which is herein incorporated by reference in its entirety to the extent that it does not conflict with the present disclosure.
Vane pumps traditionally include, among other things, a housing, a cam member and a rotor supported within the housing by axially opposed journal bearings. The housing defines an interior chamber, a fluid inlet and a fluid outlet and the cam member and rotor are disposed within the interior chamber. The cam member has a central bore which defines the circumferential boundary of the internal pumping chamber. Mounted for rotational movement within the central bore of the cam member, is a rotor supported by axial opposed journal bearings. The rotor element has circumferentially spaced apart slots machined therein which support corresponding radially movable vane elements.
Variable displacement vane pumps differ from other vane pumps, such as fixed displacement vane pumps, in that the cam member pivots about a fulcrum aligned with the vertical centerline of the pump, thereby adjusting its position with respect to the rotor. This adjustment allows the relative volumes of the inlet and discharge buckets to be changed and thereby vary the displacement capacity of the pump.
In a single rotation, the vanes of the rotor element of the pump traverse at least four distinct arcuate regions which make up the 360 degree revolution. The first region is the inlet arc segment in which fluid is received into the pumping chamber and over this region the bucket volume increases. The second region is the discharge arc segment in which pressurized fluid is discharged from the pumping chamber and over this region the bucket volume decrease. Lastly, seal arc segments separate the inlet and discharge arc segments and represent the regions through which the bucket volume remains substantially constant.
In operation, fluid at a first pressure is fed into the pumping chamber through the housing inlet, and into the space defined between adjacent vane elements, known as the bucket. In positive displacement vane pumps, as the vane elements rotate within the pumping chamber from the inlet region to the outlet region, the configuration of the cam member causes the vanes to retract within the corresponding slots. This causes the volume defined by the bucket to decrease. Since the amount of fluid received into an inlet bucket is greater than that contained within the corresponding discharge bucket, a fluid volume equivalent in size to the volumetric difference is discharged or displaced through the outlet port at a pressure equal to the downstream pressure which must be overcome.
Typically, pumping pressures and velocities are so high within the pump housing that the use of heavy, high wear resistant materials for the cam member and the vane elements becomes necessary to handle the wear which is caused by these high levels of pressure and velocity.
Prior variable displacement vane pumps are illustrated in U.S. Pat. No. 5,545,014 to Sundberg et al. and U.S. Pat. No. 5,833,438 to Sundberg. U.S. Pat. No. 5,545,014 discloses a durable, single action, variable displacement vane pump capable of undervane pumping and a pressure balancing method. U.S. Pat. No. 5,833,438 to Sundberg teaches a variable displacement vane pump having a durable rotor member with journal ends at each side of a large diameter central vane section and a mechanism for confining the high pressure within the cam member and thereby preventing axial pressure leakage along the length of the rotor member. The disclosure contained within these patents is hereby incorporated by reference in their entirety to the extent it does not conflict with the present disclosure.
The advantages of variable displacement pumps over conventional pumps, namely gear pumps, is that they solve the problem where excess heat generation becomes a crucial impediment to pump performance. Also, a variable displacement vane pump can be used to eliminate certain fuel flow metering components by utilizing the pump as the metering device.
One of the disadvantages associated with vane pump technology is the failure mode. As a result, there is a reluctance to implement this technology in applications, such as high performance aircraft, that require high operational reliability and a predicted failure mode. With a conventional gear pump, the failure mechanism is well known. Typically as the pump degrades, the performance drops off far enough so that eventually one cannot start the engine, thus a safe failure occurs. With a vane pump, however, as the vanes wear away due to contact with the cam surface, the cantilevered load that the pressure puts on each vane can become so high that a catastrophic failure of a vane can occur during pump operation and effectively destroys the whole pumping system without warning. In applications such as helicopter fuel systems, this type of failure can cause damage to the control system and engine. In order to prevent such an occurrence, the vane pump must be inspected and maintained frequently.
In view of the foregoing, a need exists for an improved vane pump which resembles the failure mode of a gear pump by “tracking” wear of the vanes, and disabling the engine from starting after a certain level of wear is attained.